Dough:
Mix all ingredients but butter in mixer, rest 10 mins
Mix again and add butter, transfer ball to container and cut cross in the top
Ferment room temp until 1.5x size then fridge for 4-12h
Deflate dough, stretch and flatten into rough square, wrap in cling film and roll into neat 8 inch square. Freeze 20 mins
Roll dough to 8" x 16", place butter in middle, fold dough over and rotate 90 degrees
Roll to a 24" long rectangle ½" thick, trim edges, do book fold. Freeze 15 mins then fridge 1h
Roll into long slab ⅜" thick, do letter fold. Freeze 15 mins, fridge 1h
Roll to 14" x 17" slab, wrap in plastic, freeze 20 mins, fridge until morning
Prep oven with pan of steaming water
Roll out dough if necessary, cut shorter sides, trim irregular areas, then cut into 4 long rectangles
Slice each rectangle into 2 triangles, grab corners, tug gently to widen and lengthen, then roll up, resting on point of triangle
Proof until 2x size, then fridge for 20 mins, eggwash and bake at 190c for 20 mins, swap/rotate trays and bake 10-15 mins more